1. Field of the Invention
The present invention relates to a controller for a drive system, and in particular to a technique of storing electric power generated under regenerative braking of a vehicle.
2. Description of the Background Art
In recent years, as part of measures to address environmental problems, a hybrid vehicle that runs by driving force obtained from at least one of an engine and a motor has received attention. A battery, a capacitor (condenser), and the like are mounted on such a hybrid vehicle for storing electric power to be supplied to the motor.
Japanese Patent Laying-Open No. 08-296537 discloses a retarder device that charges a capacitor and a battery with electric power regenerated under braking. The retarder device described in Japanese Patent Laying-Open No. 08-296537 includes a motor/generator connected to a mechanically-driven system, and a battery connected to the motor/generator, and allows the motor/generator to serve as a power generator under braking to regenerate electric power. A capacitor is further connected to the motor/generator for storing the regenerated electric power. The retarder device includes a charge control portion that supplies the regenerated electric power to the battery for charging the same such that the voltage between terminals of the battery reaches or exceeds the gassing voltage under braking. The charge control portion charges the battery when the voltage between terminals of the capacitor reaches or exceeds predetermined allowable voltage.
According to the retarder device described in the document, electric power regenerated under braking is utilized to provide a refresh charge such that the voltage between the terminals of the battery reaches or exceeds the gassing voltage. It is thereby possible to prolong the life of the battery without supplying charging power from the outside of the vehicle. Furthermore, the regenerated electric power is also supplied to the capacitor under braking, and when the voltage between the terminals of the capacitor reaches or exceeds a permissible value, surplus power is supplied to the battery for a refresh charge. It is thereby possible to utilize the surplus regenerated electric power to perform a refresh charge of the battery.
As to the retarder device described in Japanese Patent Laying-Open No. 08-296537 , electric power stored in the capacitor and the battery is used to allow the motor/generator to serve as an electric motor to provide torque assist at the time of engine start and acceleration. However, under regenerative braking, it is not always possible to obtain voltage that reaches or exceeds a permissible value between the terminals of the capacitor. In other words, it is not always possible to obtain regenerative electric power with which the capacitor is fully charged. In such a case, it may not be possible to store sufficient electric power required to accelerate a vehicle as demanded by a driver under reacceleration after regenerative braking.